Did a planetary wobble kill the dinosaurs?

A wobble in Mercury's orbit could have wiped out the dinosaurs. Computer simulations show that the planet's orbit wobbled 65 million years ago.

This could have been just the thing to send an asteroid hurtling towards Earth, resulting in the huge impact that caused the mass extinction at the end of the Cretaceous period, says Bruce Runnegar from the University of California at Los Angeles' Center for Astrobiology.

But other researchers disagree. Mark Bailey, director of the Armagh Observatory in Northern Ireland, says that the theory relies on an unlikely chain of events. "I can't believe that Mercury has an effect on anything in the Solar System. It's such a small planet."

Knock-on effect

Runnegar and his colleagues used computer models to map out the Solar System for the past 250 million years. In particular, they looked at the perihelion of each planet - the point in its orbit where it is closest to the Sun.

The perihelion of Earth rotates around the Sun with a period of hundreds of thousands of years. Because of subtle tugs and pulls between the planets, this period changes slightly with time.

Now Runnegar and his colleagues have found that these changes in the Earth's orbit had a knock-on effect on the inner planets, they told an Earth Systems Processes meeting in Edinburgh.

Their model suggests one of these blips significantly changed Mercury's orbit 65 million years ago. This wobble would have pulled at the asteroid belt, increasing the chances that asteroids in the Hungarias region would be knocked out of place. Now the researchers are running a fresh set of models to see how much the orbits of these asteroids changed.

Single bullet

It wouldn't have been enough to send a shower of asteroids into the Earth, but Runnegar says the wobble could have sent a single asteroid onto collision course with our planet. "It's quite plausible that some of the asteroids were affected," he says.

Bailey says that the group's modelling is admirable. But the link with the dinosaurs is "tenuous to say the least", he says. "It's a great long chain of 'possiblys'."

Runnegar accepts that it's extremely hard to model the workings of the Solar System many millions of years ago, particularly since the planets seem to move more chaotically the further back you go. "It looks complicated, and it is," he says.

But he and his team have tested the accuracy of their model by making tiny changes to the initial positions of the planets and rewinding backwards through time. They found the results were similar enough to be reliable to 70 million years. "We achieved significantly better accuracy than previous work," says Runnegar.

Now he is planning to run his models forward in time, to see when the next potentially catastrophic planetary wobble will be.

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